1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and, more particularly, to a liquid crystal display apparatus suitably used for motion image displaying, and a method for driving the same.
2. Description of the Related Art
The liquid crystal display apparatus have widely been used as display units for mobile devices represented by desktop and notebook type personal computers, a portable telephone set and the like. Recently, with increased demands for smaller market space and lower consumption of power, attention has been focused on liquid crystal television as a replacement for a cathode ray tube (CRT). Compared with the display unit such as the CRT, the liquid crystal display apparatus exhibits more excellent performance including reductions in size, weight and consumption of power, an increase in definition and the like. In the case of a low-speed motion image, in which an object to be displayed moves slowly, display performance is substantially equal to that of the CRT. However, in the case of a high-speed motion image, in which an object quickly moves, for example a sports program, image blurring, a contrast reduction slightly lowering image definition, and other problems occur.
For displaying of the liquid crystal display apparatus, in addition to a mainstream twisted nematic (TN) principle, in-plane switching (IPS) characterized by a wide angle of view, a multidomain vertical alignment (MVA) and the like have been used. In any case, an image is formed by making an illumination light from an illuminator (alias backlight) installed on the backside of the display unit incident on a liquid crystal panel capable of controlling a light transmissivity by rotating liquid crystal molecules according to an applied voltage. In such a conventional liquid crystal display apparatus, a cause of motion image blurring is considered to be a combination of a liquid crystal response speed and hold displaying common to the liquid crystal display apparatus and a plasma display apparatus. As the illuminator of the conventional liquid crystal display apparatus is always lit, when a displayed image is changed every moment as in the case of a motion image, a transient state of a transmissivity change before a sufficient optical response of a liquid crystal to written image data is also displayed. Consequently, a blurred image is detected by human eyes. In addition, in an always lit state of the illuminator, an image displayed in a given frame is held until a moment of next frame rewriting. Such a display system is called a hold display system. Blurring of a motion image caused by mismatching between the hold display system and human visual performance is described in pp. 13 to 18 of “Technical Report IDY 2000-147 of Institute of Image Information Media Engineers”, September, 2000. This Report also describes a technology for intermittently light an illuminator to correct motion image blurring caused by a liquid crystal response or mismatching between the hold display system and the human visual performance. Specifically, it is described that a rate (lighting duty) of lighting the illuminator in a period of one frame affects a quality of a motion image, this lighting duty must be set equal to/lower than ½ when a motion image moved at a normal speed is displayed by using a high-speed response liquid crystal display (permissible limit of motion image blurring), and a detection limit, human eyes being unable to detect motion image blurring beyond this limit, is reached when the lighting duty is lowered to about ¼.
A level of improvement of the motion image with respect to the lighting duty depends on a moving speed of the motion image. Studies by the inventors et al have revealed that in the case of a low-speed image, a good motion image below the detection limit can be obtained even at a lighting duty of about ½. Moreover, Japanese Patent A-2000-293142 discloses a technology for improving motion image display performance of a liquid crystal display apparatus by intermittently lighting an illuminator.
To display an image by intermittent lighting of the illuminator, it is necessary to separate a scanning period for writing image data in the image from a lighting period of the illuminator. That is, the illuminator is basically lit after completion of an optical response of a liquid crystal corresponding to the image data written during scanning.
FIGS. 2A and 2B are explanatory views clarifying problems inherent in the liquid crystal display apparatus by intermittent lighting, assuming a case of black and white displaying on a full screen for each frame. FIG. 2A shows a display sequence, and optical responses of liquid crystals in a first line as an uppermost line, in an n-th line as a center line, and in a 2n-th line as a lowermost line on a screen in a light period 301. FIG. 2B shows a distribution of luminance in a longitudinal position when an image to be displayed white on a full screen is written. When a conventional scanning method for writing image data sequentially from an upper side to a lower side of the screen is used as shown in FIG. 2A, screen luminance is reduced from the upper side to the lower side of the screen as shown in FIG. 2B, and thus luminance inclination of the image is recognized.
Such luminance inclination occurs because of a writing operation of an active matrix, and intermittent light of the illuminator. Therefore, a displaying principle of a liquid crystal display apparatus of an active matrix type is now described.
A frame frequency of a typical liquid crystal display apparatus is 60 Hz, and one frame period is about 16.7 ms (milli-sec.). A phenomenon of reaching a corresponding light transmissivity after a voltage is applied to a liquid crystal is called an optical response of the liquid crystal, and a period from the voltage application to exhibition of the light transmissivity corresponding to the applied voltage by the liquid crystal is called an optical response period of the liquid crystal, normally indicating a time necessary for an optical response change from a transmissivity of 10% to 90% or 90% to 10%. Here, an example of a liquid crystal display material having an optical response characteristic of 8 ms. Scanning means selection of one line, and writing of image data in this line on all the screens. A period until an end of scanning is called a scanning period. A period of selecting one line, and writing image data of a pixel of this line is called a selection period. Writing of the image data in the pixel means application of a voltage to a liquid crystal carried out such that the liquid crystal can exhibit a desired transmissivity.
FIG. 3 shows an equivalent circuit of the active matrix liquid crystal display apparatus. At a starting time of the selection period, a potential for turning ON an active element 203 is applied to a line wiring 201 by a gate driver 196. A potential dependent on image data is applied to a column wiring 202 by a drain driver 107. A potential dependent on image data is applied to a pixel electrode 210 through the active element 203. A difference in potentials between the pixel electrode 210 and a common electrode 204 is charged to a liquid crystal 208 and a holding capacitor 205 connected in parallel. At an end time of the selection period, a potential for turning OFF the active element 203 is applied to the line wiring 201, completing the writing. The charging of the liquid crystal 208 and the holding capacitor 205 is finished within a very short time compared with an optical response of the liquid crystal. In this case, a light transmissivity exhibited by the liquid crystal 208 corresponds to an absolute value of an applied voltage, not dependent on polarity of the voltage.
Now, description is made of flickers and polarity of an applied voltage by referring to FIGS. 4A to 4D. It is generally known that liquid crystal property is deteriorated when a DC voltage is applied. In the case of image data supplied to a liquid crystal of a given pixel, normally, its polarity must be reversed at least for each frame. A transmissivity of the liquid crystal is decided by a size of an applied voltage, not dependent on its polarity. However, in the case of driving by using the active element, because of effects of parasitic capacitance of the active element or a leakage current in an OFF state of the active element, even if a potential is supplied from the data driver to apply a voltage of an equal size to the common electrode 204, slight deviation occurs in a value of a voltage actually applied to the liquid crystal. Consequently, because of a difference in luminance between positive and negative polarities even in the same image data, flickers are recognized at a frequency of about 60 Hz. For suppressing flickers, there are a method of increasing a frame frequency, and reversing positive and negative polarities at a frequency, at which human eyes cannot recognize a luminance difference between the positive and negative polarities, a method of preventing flickers from being recognized by human eyes by spatially dispersing pixels written at positive and negative polarities so as to average luminance differences, a method of using only a single polarity for displaying by lighting an illumination light source only at one of positive and negative polarities, at which writing is displayed. Conventionally, because of limited driving capabilities of the gate driver and the data driver, and in order to prevent a reduction in luminance caused by single polarity displaying, especially in the case of a large liquid crystal display apparatus, the method of spatially dispersing writing polarities has mainly been used. FIGS. 4A to 4D show polarities of image data written in pixels. Specifically, FIG. 4A shows a driving system for reversing polarities for each frame without spatially dispersing polarities of an applied voltage, which is called frame reversal driving; FIG. 4B a driving system for reversing polarities of an applied voltage for each line, and then reversing the polarities for each frame, which is called each-line reversal driving; FIG. 4C a driving system for reversing polarities of an applied voltage for each column, and then reversing the polarities for each frame, which is called each-column reversal driving; and FIG. 4D a driving system for reversing polarities of an applied voltage for each line and column, and then reversing the polarities for each frame, which is called dot reversal driving.
The frame reversal driving shown in FIG. 4A is designed to write image data of similar polarities on a full screen surface, and advantageous in that a potential outputted by the data driver in a given frame can always bet set equal to that of a common electrode, and a low withstand pressure data driver cab be used by combining the system with a common AC driving system for changing a potential of the common electrode 204 according to a writing polarity. However, when polarities of a displayed image made visible are simply reversed for each frame at a frame frequency of 60 Hz, flicker may be recognized because of a difference in writing characteristics between the positive and negative polarities.
In the cases of the each-line reversal driving shown in FIG. 4B and the each-column reversal driving shown in FIG. 4C, flickers can be prevented from being recognized by dispersing polarities of displayed images on screens, and averaging and displaying luminance differences caused by polarity differences to human eyes. In the case of the dot reversal driving shown in FIG. 4D, since polarities of a displayed image are reversed for each line, and then for each column, luminance differences caused by polarity differences are more averaged, thus preventing recognition of flickers.
When write scanning is performed from the upper side to the lower side of the screen as shown in FIG. 2A, writing is executed in the 1st line at the starting time of a scanning period; in the n-th line at the middle time of the scanning period; and in the 2n-th line at the end time of the starting period. Thus, since starting of liquid crystal optical response varies depending on a position within the screen, when the same image data is written on the full surface of the screen, a completion time of optical response also varies depending on a position in the screen. As shown in FIG. 2A, the process is carried out based on a sequence, where scanning is finished at a ½ frame, and the illuminator is lit in a ¼ frame period of a latter half. However, since image data is written in a pixel of the 1st line as an uppermost line of the screen at a starting time of the frame, liquid crystal optical response is completed with 8 ms from the frame starting time. On the other hand, image data is written in a pixel of the n-th line in the center of screen with 4 ms from the frame starting time; image data is written in a pixel of the 2n-th line in the lowermost side of the screen with 8 ms from the frame starting time; and each liquid crystal response is completed with 16 ms from the frame starting time. Here, as the illuminator is lit with 12 ms from the frame starting time, the liquid crystal optical response is completed in the pixel of the n-th line. But the liquid crystal optical response is not sufficiently completed in pixels of lines lower than the n-th. If the illuminator is lit in an uncompleted state of liquid crystal optical response, luminance is reduced in the case of white displaying, causing luminance inclination. FIG. 2B shows dependence of luminance inclination on a longitudinal position. In the foregoing, the optical response time of the liquid crystal was 8 ms, which was in the case of the liquid crystal having a relatively fast response speed. However, in a current liquid crystal display apparatus, an optical response time of a liquid crystal often exceeds even 20 ms. When a liquid crystal having such a slow response speed is used, a reduction may probably occur in luminance starting from the upper side of the screen, not from the center. For a TV image, a motion image appears in the vicinity of the screen in most cases, which is considered to be an area, where a view point of a viewer concentrates. Therefore, considering the view point concentration area of the viewer, when even slight luminance inclination occurs, luminance must be set highest in the vicinity of the center.
On the other hand, Japanese Patent A-11-237606 discloses a method of reversing upper and lower scanning directions for each field, in order to suppress luminance inclination dependent on a longitudinal position. However, since this method uses interlaced driving, when motion image data of one field is simply converted from field data into frame data, a DC component may be superimposed.
Furthermore, as methods for canceling an effect of display history of a previous frame, the above-described publication discloses a method of applying a preset voltage, and a method of applying positive and negative data signal voltages after application of a preset voltage.
FIG. 26 is an explanatory view showing a problem of properties when preset voltages are cyclically applied en block on the full surface of the screen according to the above-described method. In the drawing, writing voltages Vs1, Vsnm, and VS2n in the pixels of the 1st line in the uppermost layer, the n-th line in the center and the 2n-th line in the lowermost layer for two frames, and liquid crystal optical responses T1, Tn and T2n of the respective pixels are shown. In the line of the uppermost layer, since image data having a polarity reversed is written immediately after the application of a preset voltage, and at the middle time of the application of a next preset voltage, AC driving is achieved, where effective values of positive and negative voltages applied to the liquid crystal are equal to each other. However, rates of positive and negative voltage application time become more asymmetrical toward the lower side of the display area, and a DC voltage is effectively applied. In the lowermost layer, asymmetry becomes most conspicuous, bringing about one-side polarity driving. Consequently, it is difficult to suppress occurrence of flickers caused by superimposition of a DV voltage, and to achieve displaying of a motion image without any residual images. Therefore, there is a demand for an in-frame AC driving method for achieving liquid crystal AC driving in one frame irrespective of images or a displaying position in the panel.
In addition, a method may be employed, which apples preset voltages sequentially for lines in synchronization with scanning by dividing one frame into three parts, and setting a ⅓ frame as a resetting period. In this case, however, a certain writing operation is carried out during intermittent lighting of the illuminator, unfavorable crosstalk may be generated through a parasitic capacity between the line or column wiring and the pixel.